The present document is based on Japanese Priority Document JP 2001-067455, filed in the Japanese Patent Office on Mar. 9, 2001, the entire contents of which being incorporated herein by reference.
1. Field of the Invention
The present invention relates to a resonance type power supply unit, with a switching circuit having an inductive element and a capacitive element, which causes a current waveform or a voltage waveform to resonate.
2. Description of the Related Art
Conventionally, there have been widely in use a power source unit of a inverter type which switches a direct current obtained by rectifying and smoothing a commercial alternating current and outputs an alternating current and a switching power source designed to convert an inverter output with high efficiency to the desired voltage by means of a transformer. In a power supply unit using the power source of the inverter type or the switching power source as those described above, an output power control is performed by any of the methods such as a pulse width modulation (PWM) method for detecting the output voltage and current, providing the power by multiplying the values obtained with the integrator, and controlling the duty factor of the switching pulse; a frequency control method which controls the frequency and phase of the switching pulse, and a phase control method.
Also, overcurrent protection is typically used as the conventional means of protecting the power supply unit. The current has been detected by a detection resistance, a current transformer, a Hall element or the like.
Further, as a conventional method for suppressing output variation due to voltage variation in a power source, as disclosed in Japanese Patent Application Publication Hei 9-120221, for example, there has been proposed a method of controlling an output of a high-frequency power circuit in proportion to a difference from a second reference voltage which changes proportionally to the power voltage supplied to the above-mentioned high-frequency power circuit.
When the power control of output and overcurrent protection are performed in the conventional power supply unit, a detection resistance is typically used for current detection, so that in a case of a large current, a loss necessarily becomes large. Also, the control circuit for power control calls for a complicated configuration, thus increasing the number of parts and giving rise to such problems as a resultant decrease in reliability and a cost increase. Further, if the arrangements are made to detect a current with a current transformer, a Hall element, or the like, an increase in cost cannot be avoided.
In view of the foregoing conventional problems, the present invention provides a resonance type power supply unit which can perform power control of output and overload protection easily at low cost and at low loss.
According to the present invention, there is provided a resonance type power supply unit having an inductive element and a capacitive element in its switching circuit and causing a current waveform or a voltage waveform to resonate. The unit comprises a resonance voltage detector for detecting a resonance voltage by the inductive element and the capacitive element, a voltage converter for converting the resonance voltage detected by the resonance voltage detector to a comparable DC voltage, a voltage comparator for comparing a DC voltage obtained by the voltage converter to a reference voltage, and a switching controller for controlling a switching frequency or an on-state time interval of the switching circuit to cause the DC voltage to correspond to the reference voltage based on an comparison output by the voltage comparator.
As apparent from the foregoing, in the resonance type power supply unit according to the present invention having an inductive element and a capacitive element in its switching circuit and causing a current waveform or a voltage waveform to resonate, it is possible to control the output power at the constant level by controlling the switching frequency or the on-state time interval of the switching circuit so that the resonance voltage due to the inductive element and capacitive element may reach the preset voltage.
Also, in the resonance type power supply unit according to the present invention, the output power can be easily controlled by changing the reference voltage which provides the above-mentioned preset voltage.
Further, in the resonance type power supply unit according to the present invention, soft start control and soft stop control for protection of the constituent elements as well as anti-flickering measures of the AC line can be easily accomplished by increasing the reference voltage gradually to the preset value at the time of the start control, decreasing the reference voltage gradually to the preset value at the time of the stop control, and stopping the switching of the switching circuit.
Furthermore, in the resonance type power supply unit according to the present invention, since the resonance voltage of the inductive element and capacitive element reaches a point approximately in proportion to the output power, the overload protection measure can be easily applied by detecting the resonance voltage.
Therefore, according to the present invention, it is possible to provide the resonance type power supply unit which can perform the power control of output and overload protection easily at a low cost and at low loss.